The invention relates to a light ledge for a level or gauge glass. Gauge or level glasses for liquids may advantageously be read against a background illumination, and it is known to mount a light source along one side of a level glass and read the liquid level from the opposite side. Level glasses are frequently mounted in process plants where explosion risks exist and, therefore, very high standards are required of the design of light sources. Good light ledges for shining through level glasses may advantageously consist of light emitting diodes mounted in a row. Light emitting diodes are stout and rugged, use little current and have a long useful life compared with electrical bulbs. For use in explosion hazardous areas, light emitting diodes having current feed, must be cast into a gas-tight casting mass.
Upon casting-in, one should avoid that casting mass covers the lighting part of the light emitting diodes. It is known to attach short pipes of plastic or metal with one end tight against a glass ledge and let said pipes serve as carrier for light emitting diodes which are fixed to and electrically coupled on a circuit board. Often, the pipes have a reflecting inner surface and may be funnel-shaped, each pipe, in addition to a casting formwork, serving as a light reflector. Upon mounting, the circuit board with the diodes is moved towards the glass ledge, the pipes orientated such that each light emitting diode slides into the open end of a pipe and becomes covered thereby. During casting-in or encapsulating, the pipes serve as formworks, preventing casting mass from penetrating inwards to the lighting part of the light emitting diodes. Light may thereby pass unimpeded from the light emitting diode within each pipe and out through the glass ledge.
A disadvantage of the construction described is that the manufacture is time-consuming and expensive. Each pipe, which is to cover a light emitting diode, must be sealed completely into the light emitting diode, requiring that the pipe is split and formed in a resilient material. Each pipe must be mounted beforehand on the glass ledge, e.g. with glue. Each light emitting diode must be passed so far into the pipe that the split is covered, in order to avoid that casting mass penetrate into the pipe. Again, this requires accurate mounting of the light emitting diodes on the circuit board.
The object of this invention is to provide a simple encapsulatable light ledge having light emitting diodes, and wherein said disadvantages are avoided.
According to the invention, the object is achieved through the features defined in the following claims. An exemplary embodiment of the invention is described in the following, reference being made to the attached drawings.